1. Field of the Invention
This invention relates generally to the field of adjustable scaffolds comprising a platform, a base, and an adjustable interconnecting support structure. It relates specifically to an adjustable scaffold which efficiently and safely lifts heavy loads with a minimum of stress on its supporting members. The scaffold can be raised and lowered to a range of heights intermediate stored and elevated positions, and provides a stable scaffold at each of those heights.
2. Description of the Prior Art
In building construction and overhead maintenance, for example, it is often necessary to provide adjustable scaffolds or work towers for workmen and equipment. In the past most adjustable scaffolds comprised a platform, a base, and an adjustable, interconnecting structure, typically including "lazy tong" or scissors connectors. Each set of scissors connectors comprised at least a pair of crossed members of substantially the same length, pivotally connected near the center of each member. The scissors connectors were movable to elevated and stored positions by lifting devices, typically hydraulic cylinders and accompanying lever brackets, so positioned that the forces exerted by the hydraulic cylinders upon the scissors connectors are always inclined at an angle from the horizontal.
The hydraulic cylinders were typically positioned so that they operated within a set of scissors connectors, usually within the bottom set. In other words, the hydraulic cylinders typically joined crossed members from the same set of scissors connectors. This placement required that one end of the hydraulic cylinder be attached near the lower end of a member of the scissors connectors, and the other end of the cylinder be attached near the center of the other member, substantially as shown in Smith, Jr., U.S. Pat. No. 3,485,321 (issued to applicant), at FIGS. 4, 5, and 7. In the past, all the hydraulic cylinder lifting means have typically been placed on one side of the plane defined by the points comprising the central pivot points of the crossed members.
Furthermore, to ensure stability, each set of scissors connectors in the prior art had two pairs of pivotally connected crossed members, each pair being positioned at opposite sides of a rectangular platform or base. Lateral supports between the two pairs were used to form a more solid structure. These lateral supports were attached at each end of the members, and at their central pivot connections, spanning the lateral gap between the two pairs of scissors connectors. The lifting means were positioned within this lateral gap, requiring support bars for connection to the outer members.
While this positioning of the cylinders and the structure of the scissors connectors has been adequate for loads of up to approximately 1500 pounds, the lifting and rigid support of heavier loads has not been satisfactorily accomplished under the prior art. Use of larger cylinders, heavier crossed members, and additional lateral supports is expensive and increases the total load which the cylinders must lift and support. Moreover, the increased stresses and torsional loads on the crossed members and pivot points of the scissors connectors increases the possibility of mechanical failure. Furthermore, positioning the lifting means on support bars at a substantial lateral distance from the crossed members exerts undesirable torques upon the crossed members at their connections to the lifting means.